Legally, long guns can be carried in your vehicle around the USA. You can even carry it locked up thru states and cities with severe restrictions...
Brent
That is true for the most part, but New Yuk State for instance, will arrest you and charge you with a felony for possessing a firearm without a permit. I went thru this whole thing 18 months ago, wanting to take a firearm with me when I flew back to help my brother move. Was gonna be a 1,400 mile trip by car, and although I didnt really wanna take a gun thru the airlines, I didnt wanna be traveling unarmed. Called and talked to state police in New Yuk, to get info on what how to go about it legally. And what it boils down to, is that to possess a fire arm in New Yuk, you must have a firearms permit, however, NY does don't have a provision for non-resident firearms permits, so a non-resident, posessing a firearm of any type, (even though federal law allows you to transport your wepon) upon crossing into NY State (be it on foot, by car, plane, or horseback) you are now commiting a felony. They even have random sting operations at the airports, where they wait for you to pick up your firearm from the airline, at which point you have now taken possession or the weapon from the airline, and they arrest you and charge you with a felony. The officers I spoke with said it would not matter if it was a long gun or pistol, as both were illegal for a non-resident with no permit.....which dosent exist anyway.
All the above info was from speaking with a couple very nice, and knowledgeable police officers (both of whom were "gun guys"), who agreed with me when I commented on the stupidty of the laws, and stated they hate to enforce those laws they don't agree with, but it was part of the job, and they hoped they wouldn't be arresting me anytime soon.
From what I have found, New York is the only state that will not allow transportation of a firearm by a non-resident, but it never hurts to check up on the places you are passin thru.